In a secondary battery such as a lithium-ion battery, a lid body made of a metal plate is welded to a circumference of an opening of a battery case containing an electrolyte therein in order to cover the opening of the battery case. In recent years, such a secondary battery is being used in an application to be mounted in a hybrid car or the like. Accordingly, reducing the weight of such a secondary battery is desired and the lid body conventionally made of a stainless steel is therefore formed now from a metal plate made of an aluminum alloy. The aluminum alloy, however, has a modulus of longitudinal elasticity smaller than that of the stainless steel and such a lid body is therefore more likely to bend outwardly due to an internal pressure thereagainst. Thus, the lid body needs to ensure a rigidity thereof by having a given thickness or greater. Furthermore, in order to completely seal the battery case so as to prevent a liquid leakage and in order to firmly fix the lid body to the battery case by means of welding, a welding margin with the battery case needs to be provided as long as possible in the lid body along a thickness direction thereof. For these reasons, the thickness of the lid body made of the aluminum alloy is set at about 1 mm.
On the other hand, the secondary battery has a risk of explosion due to an abnormal rise in an internal pressure inside the battery by heat or impact. Accordingly, an explosion-proof valve which will be broken at a predetermined internal pressure is formed beforehand in a portion of the lid body. The explosion-proof valve is edged with a thin-walled portion having a thickness smaller than that of the metal plate forming the lid body therearound. The thickness of the thin-walled portion is set so as to be broken when an internal pressure in the battery case is increased to the set internal pressure. In order to cause the breaking of the thin-walled portion at an internal pressure of 0.7 to 1 MPa, for example, the thickness of the thin-walled portion is set at 30 μm.
As a processing method for forming a thin-walled portion having a set thickness in a portion of a metal base plate having a predetermined thickness, a processing method (first processing method) as follows has been conventionally known (Patent Literature 1). Specifically, as shown in FIG. 11, upper and lower dies 101 and 102 are set in an overlapping manner with a predetermined clearance δ therebetween. A metal base plate 110 is subjected to press processing by the upper and lower dies 101 and 102 with overlapped shoulder portions 101a and 102a being used as curved surfaces R for extrusion molding. The thin-walled portion 111 is thus formed while pushing out a metal material in the portion subjected to the press processing toward both sides thereof.
As another processing method for forming a thin-walled portion having a set thickness in a portion of a metal base plate, a processing method (second processing method) as follows has also been known (Patent Literature 2). Specifically, as shown in FIG. 12, a metal base plate 120 is placed on a flat portion of a lower die 126. Sharpened blades of an upper die 125 each machined in a V-letter shape are pressed into the metal base plate 120 from above, thereby forming thin-walled portions 121 and 122 having a set thickness at lowermost ends of resultant recessed grooves each formed to have a V letter-shaped cross section.
The second processing method is a processing method for forming a thin-walled portion having a set thickness in an explosion-proof valve in a battery case lid made of an aluminum alloy plate. A first thin plate portion 120 having a thickness of 0.3 mm is formed by stretching a portion of a lid body 130 made of an aluminum alloy and having a thickness of 1 to 3 mm. The first thin plate portion 120 is interposed between the punch 125 and the die 126 having V letter-shaped protrusions, thereby forming the thin-walled portions 121 and 122 having a set thickness of about 50 μm in the first thin plate portion 120 (see FIG. 13). Excess portions of the first thin plate portion 120 resulting from pressing the V letter-shaped protrusions therein are pushed out toward release portions 123 and 124 at both the sides thereof.